Adding a reinforcing rib to STL model with blender
In this particular case, I think it can also be done in this way:

put the original mesh in front view, wireframe shading,in edit mode, 
then select just the central array of vertices, 



then duplicate (shift-D) and separate (P => "selection") those vertices (to a new mesh)

as you see in side view, you now have a "section" object made of two "loops".



deleting the "inner" vertices from both loops, you can then make a single "face" joining the remaining vertices on their edges in pairs (upper and lower) using F then, after selecting all of them, make it a big "face" pressing F



now you can extrude this face on left and right (I would not bother using mirroring), obtaining a solid central shape that perfectly follows you original mesh curvature. 



Then I would join both objects, and export to .stl again.